As the only couple with a young child in my group of friends, I understand how Mark and Mary feel. Feeling left out is normal when your life changes significantly and no one else's does. Yes, they had a child by choice but telling them "deal with it" isn't how true friendships work.
You need to be honest with them and let them know why you don't enjoy hanging out at their house. While they can't expect that their friends will cater to their new life 100% of the time, it certainly isn't unreasonable for their friends to cater to their new life once in a while. And I'm sure there's room for compromise on both sides. They can probably get themselves to the point of hiring a babysitter for outings they really want to attend and you guys can hang out their house every now and then so they don't have to get a babysitter...and yes, you guys can also not hang out sometimes.
Sure, they'll probably eventually fall into a new group of friends who are more readily able to socialize with them the way they feel comfortable and that's ok too. I just think/hope that you (your general group of friends) would make some effort to keep the friendship you've had up to this point in tact.
If Mary is feeling particularly down and left out, why not invite her out for an all girls night and then trade off with the boys at a later date so her husband can have some fun too. And yeah, if they want to hang out like old times together, then they need to break down and get a babysitter. Maybe if you guys had some open and honest conversation about what's going on, they will be more ready to see how important it is that they do that.